The present invention relates to an insulated wall structure useful in appliances such as refrigerators, freezers, or boiler units, for example. More specifically, this invention relates to an insulated wall structure containing a thermoplastic liner, a foamed-in-place foam insulation material, and a multilayer protective film between the thermoplastic liner and the foam insulation material.
A conventional insulated wall structure typically contains a plastic liner made of high impact polystyrene ("HIPS") or acrylonitrile-butadiene-styrene ("ABS") and foamed-in-place foam insulation material. The foam insulation material used in a conventional wall structure is typically a polyurethane foam insulation using a chloroflorocarbon ("CFC") blowing agent.
The CFC blowing agents can diffuse from the foam insulation material into the atmosphere. Environmental concerns surrounding the use of CFCs has caused appliance manufacturers to seek alternatives to CFCs as blowing agents for the production of polyurethane foam insulation. One such alternative is hydrochlorofluorocarbon ("HCFC") blowing agents including, for example, dichlorotrifluoroethane ("HCFC-123") and dichlorofluoroethane ("HCFC-141b").
However, there are currently problems surrounding the use of some of these alternatives to CFCs as blowing agents. For example, liners made from HIPS tend to blister when bonded to foam insulation material containing HCFC. HCFCs have also been shown to cause cracking in liners made from ABS. Thus, there is a need in the industry for a mechanism to protect HIPS and ABS liners from HCFC blowing agents used in the production of polyurethane foam insulation.
One approach to protecting plastic liners from attack by HCFC blowing agents used in foam insulation material is to chemically modify the composition of the resin used in making the liner. A second approach is to use a protective film between the polyurethane foam insulation containing the blowing agent and the thermoplastic liner. The present invention relates to this second approach.
It is also desirable to produce a protective film that is regrind-compatible with the thermoplastic liner. A regrind-compatible film allows scrap material produced to be reprocessed back into the thermoplastic liner without significantly affecting important physical properties of the liner. The capability of reprocessing scrap materials reduces the need for scrap materials to be discarded. Consequently, the process of manufacturing related products, such as refrigerators, becomes more economical and more environmentally friendly.
U.S. Pat. No. 4,707,401 issued to Benford ("Benford '401") discloses an insulative cabinet wall structure having a bilayer film peelably bonded to a liner. The inner layer of the bilayer film in Benford '401 contains copolymers of ethylene and vinyl acetate and the outer layer contains copolymers of ethylene and acrylic acid. Benford '401 teaches having the inner layer of the bilayer film peelably bonded to the liner to prevent stress cracking in the liner. Benford '401 does not address the problem of HIPS liner blistering caused by HCFC blowing agents. Nor does Benford '401 address the desire for having a film that is regrind-compatible with a liner.
U.S. Pat. No. 5,118,174 issued to Benford ("Benford '174") discloses an insulated cabinet wall structure having laminated to it a multilayer film comprising in laminated sequence an inner adhesive, releasable film layer for adhering to the liner, a film of high elongation material, an additional adhesive film layer, and an outer barrier layer. Thus, Benford '174 requires a high elongation material sandwiched between two separate adhesive layers. Benford '174 does not address the desire for having a film that is regrind-compatible with a liner.
Accordingly, there is still a need in the industry for a protective film that is useful for protecting liners from HCFC blowing agents without requiring multiple adhesive layers. There also remains in the industry the need for such a protective film that is regrind-compatible with HIPS liners to which it may be bonded.